- Solids Induced Pack-off
a) At the first signs of the drill string torquing up and trying to pack-off, the pump strokes should be reduced by half. This will minimise pressure trapped should the hole pack-off. Excessive pressure applied to a pack-off will aggravate the situation. If the hole cleans up, return flow to the normal rate.
b) If the string packs off, immediately stop the pumps and bleed down the standpipe pressure [NB not possible with a non-ported float valve]. When bleeding pressure down from under a pack-off, control the rate so as not to "U" tube solids into the drill string in case they plug the string.
c) Leave low pressure (<500 psi ) trapped below the pack-off. This will act as an indicator that the situation is improving should the pressure bleed off.
d) Holding a maximum of 500 psi on the standpipe and with the string hanging at its free rotating weight, start cycling the drill string up to maximum make-up torque. At this stage do not work the string up or down.
e) Continue cycling the torque, watching for pressure bleed off and returns at the shakers. If bleed off or partial circulation occurs, slowly increase pump strokes to maintain a maximum of 500 psi standpipe pressure. If circulation improves continue to increase the pump strokes.
f) If circulation cannot be regained, work the pipe between free up and free down weight. DO NOT APPLY EXCESSIVE PULLS AND SET DOWN WEIGHTS AS THIS WILL AGGRAVATE THE SITUATION (50k lb max). Whilst working the string continue to cycle the torque to stall out and maintain a maximum of 500 psi standpipe pressure.
g) DO NOT ATTEMPT TO FIRE THE JARS IN EITHER DIRECTION.
h) If circulation cannot be established increase the standpipe pressure in stages up to 1500 psi and continue to work the pipe and apply torque.
i) If the pipe is not free once full circulation is established, commence jarring operations in the opposite direction to the last pipe movement. Once the pipe is free rotate and clean the hole prior to continuing the trip.
- Unconsolidated formations
Description
An unconsolidated formation falls into the well bore because it is loosely packed
with little or no bonding between particles, pebbles or boulders. Video clip of sand sloughing
The collapse of the formation is caused by removing the supporting rock as the well is drilled. This is very similar to digging a hole in sand on the beach, the faster you dig the faster the hole collapses. It happens in a well bore when little or no filter cake is present. The un-bonded formation (sand, gravel, small river bed boulders etc.) cannot be supported by hydrostatic overbalance as the fluid simply flows into the formation. Sand or gravel then falls into the hole and packs off the drill string. The effect can be a gradual increase in drag over a number of metres, or can be sudden. This mechanism is normally associated with shallow formations. Examples are shallow river bed structures at about 500m in the central North Sea and in surface hole sections of land wells. This mechanism normally occurs:
· While drilling shallow unconsolidated formations.
Preventative Action
These formations need an adequate filter cake to help stabilise the formation. Seepage loss can be minimised with fine lost circulation material. If possible, avoid excessive circulating time with the BHA opposite unconsolidated formations to reduce hydraulic erosion. Spot a gel pill before POOH. Slow down tripping speed when the BHA is opposite unconsolidated formations to avoid mechanical damage.
Start and stop the pumps slowly to avoid pressure surges being applied to unconsolidated formations. Control-drill the suspected zone to allow time for the filter cake to build up, minimise annulus loading and resultant ECD’s. Use sweeps to help keep the hole clean. Be prepared for shaker, desilter and desander overloading. A method successfully used in the North Sea is to drill 10m, pull back to the top of the section and wait 10 minutes. Note
any fill on bottom when returning to drill ahead. If the fill is significant then ensure the process is repeated every 10m. It may be impossible to prevent the hole collapsing. If so let the hole stabilise itself with the BHA up out of harm’s way.
Rig site indications
· Increase in pump pressure.
· Fill on bottom.
· Overpull on connections.
· Shakers blinding.
Freeing
Follow First Actions but be aware that the pressures (i.e. 500 psi, 1500 psi) will probably not be achievable in shallow
formations.
- Mobile Formations
Description
The mobile formation squeezes into the well bore because it is being compressed by the overburden forces. Mobile formations behave in a plastic manner, deforming under pressure. The deformation results in a decrease in the well bore size, causing problems running BHA’s, logging tools and casing. A deformation occurs because the mud weight is not sufficient to prevent the formation squeezing into the well bore. This mechanism normally occurs: · While drilling salt.
Preventative Action
Maintain sufficient mud weight. Select an appropriate mud system that will not aggravate the mobile formation. Plan frequent reaming/wiper trips particularly for this section of the hole. Consider bi-centre PDC bits. Slow trip speed before BHA enters the suspected area. Minimise the open hole exposure time of these formations. With mobile salts consider using a slightly under-saturated mud system to allow a controlled washout.
Rig site indications
· Overpull when moving up, takes weight when running in.
· Sticking occurs with BHA at mobile formation depth.
· Restricted circulation with BHA at mobile formation depth.
Freeing
Spot a fresh water pill if in a salt formation. (Consider the effect on well control and on other open hole formations ). If moving up, apply torque and jar down with maximum trip load. If moving down, jar up with maximum trip load. Torque should not be applied while jarring up.
- Fractured & Faulted Formations
Description
A natural fracture system in the rock can often be found near faults. Rock near faults can be broken into large or small pieces. If they are loose they can fall into the well bore and jam the string in the hole. Even if the pieces are bonded together, impacts from the BHA due to drill string vibration can cause the formation to fall into the well bore. This type of sticking is particularly unusual in that stuck pipe can occur while drilling. When this has happened in the past, the first sign of a problem has been the string torquing up and sticking. There
is a risk of sticking in fractured / faulted formation when drilling through a fault and when drilling through fractured limestone formations.
This mechanism can occur:
· in tectonically active zones.
· in prognosed fractured limestone.
· as the formation is drilled.
Preventative Action
Minimise drill string vibration. Choose an alternative RPM or change the BHA configuration if high shock vibrations are observed. Slow the trip speed before the BHA enters a suspected fractured/faulted area. Generally, fractured formations require time to stabilise. Be prepared to spend time when initially drilling and reaming prior to making significant further
progress. Circulate the hole clean before drilling ahead. Restrict tripping speed when BHA is opposite fractured formations and fault zones. Start / stop the drill string slowly to avoid pressure surges to the well bore. Anticipate reaming during trips. Ream fractured zones cautiously.
Rig site indications.
· Hole fill on connections.
· Possible losses or gains.
· Fault damaged cavings at shakers.
· Sticking can be instantaneous.
Freeing
If packed off while off bottom then follow First Actions. Otherwise JAR UP in an effort to break up formation debris. Use every effort to maintain circulation. Circulate high density viscous sweeps to clean debris. Spot acid if stuck in limestone
- Naturally Over-Pressured Shale Collapse
Description
A naturally over-pressured shale is one with a natural pore pressure greater than the normal hydrostatic pressure gradient. Naturally over-pressured shales are most commonly caused by geological phenomena such as under-compaction, naturally removed overburden (i.e.
weathering ) and uplift. Using insufficient mud weight in these formations will cause the hole to become unstable and collapse.
This mechanism normally occurs in:
· Prognosed rapid depositional shale sequences.
Preventative action
Ensure planned mud weight is adequate. Plan to minimise hole exposure time. Rigorous use of gas levels to detect pore pressure trends. Use of other information to predict pore pressure trends (for example Dexp ). Once the shale has been exposed do not reduce the mud weight. It may also be the case that the mud weight will need to be raised with an increase in inclination See Well bore stability Section of SP KB 1997 Guidelines.
Rig site indications
· Cavings (splintery) at shakers.
· Increased torque and drag.
· Gas levels, D exponent.
· Circulation restricted or impossible.
· Hole fill.
· An increase in ROP.
· Cuttings and cavings are not hydrated or mushy.
Freeing
Follow First Actions.
- Induced Over-Pressured Shale Collapse
Description
Induced over-pressure shale occurs when the shale assumes the hydrostatic pressure of the well bore fluids after a number of days exposure to that pressure. When this is followed by no increase or a reduction in hydrostatic pressure in the well bore, the shale, which now has a higher internal pressure than the well bore, collapses in a similar manner to naturally over-pressured shale. Video clip - Unstable Shale in WBM
This mechanism normally occurs:
· In WBM.
· After a reduction in mud weight or after a long exposure time during which
the mud weight was constant.
· In the casing rat hole.
Preventative action
Non water based muds prevent inducing over-pressure in shale. Do not plan a reduction in mud weight after exposing shale. If cavings occur, utilise good hole cleaning practices. See Hole Cleaning Section
Rig site indications
· Cuttings / cavings show no sign of hydration.
· Cavings (splintery) at shakers.
· Tight hole in casing rat hole.
· Increased torque and drag.
· Circulating restricted or impossible.
· Hole fill.
1.2.6.4 Freeing
Follow First Actions.
- Reactive Formations
Description
A water sensitive shale is drilled with less inhibition than is required. The shale absorbs the water and swells into the well bore. The reaction is ‘time dependent’, as the chemical reaction takes time to occur. However, the time can range from hours to days.
This mechanism normally occurs:
· When using WBM in shales and clays in young formations.
· When drilling with an incorrect mud specification. Particularly, an
insufficient concentration of inhibition additives in OBM and WBM such as
salts (KCl, CaCL), glycol and polymer.
Preventative action
Use an inhibited mud system. Maintain the mud properties as planned. The addition of various salts (potassium, sodium, calcium, etc. ) will reduce the chemical attraction of the water to the shale. Various encapsulating (coating) polymers can be added to WBM mud to reduce water contact with the shale. Monitoring mud properties is the key to detection of this problem. Open hole time in shale should be minimised. Regular wiper trips or reaming trips may help if shales begin to swell. The frequency should be based on exposure time or warning signs of reactive shales. Ensure hole cleaning is adequate to clean excess formation i.e. clay balls, low gravity solids etc.
Rig site indications
· Hydrated or mushy cavings.
· Shakers screens blind off, clay balls form.
· Increase in LGS, filter cake thickness, PV, YP, MBT.
· An increase or fluctuations in pump pressure.
· Generally occurs while POOH.
· Circulation is impossible or highly restricted.
Freeing
POH slowly to prevent swabbing. See First Actions.
- Hole Cleaning
Description
In deviated wells cuttings and cavings settle to the low side of the hole and form layers called solids beds or cuttings beds. The BHA becomes stuck in the solids bed.
OR Cuttings and cavings slide down the annulus when the pumps are turned off and pack-off the drill string. Avalanching can also occur while the pumps are on.
Good hole cleaning means removal of sufficient solids from the well bore to allow the reasonably unhindered passage of the drill string and the casing.
There are several main reasons for solids not being cleaned out of the well bore.
These are:
· A low annular flow rate.
· Inappropriate mud properties.
· Insufficient circulation time.
· Inadequate mechanical agitation.
If any of the above are missing good hole cleaning will be very unlikely. In 40-65 degree wells the cuttings bed will slide down the low side of the hole. This can happen while pumping, not just when the pumps are off. In highly deviated wells of 65 degrees or more cuttings settle very quickly in spite of high flow rates. This is known as
avalanching. A cuttings bed of 10% of the hole diameter (1.75 inches in 17.5 inch hole) looks harmless enough. Add a drill string and the situation looks very different.
Cuttings beds can also increase drag in the well and cause problems with applying WOB in horizontal holes.
Preventative Action
· Maximise the annular velocity.
- Consider the use of a third mud pump.
- Consider using larger drill pipe.
· Ensure circulation times are adequate.
- Consult the hole cleaning charts for confirmation.
- Monitor the cuttings returns at the shakers.
· Maximise mechanical agitation of cuttings beds.
- Rotation.
- Reciprocation.
· Optimise mud properties.
- increase YP in near vertical wells.
Rig site indications· Overpulls increasing while POOH from TD in deviated hole (7-10 stands).
· Erratic pump pressure.
· Poor weight transfer to bit.
· Difficulty orienting toolface.
· Absence of returns at shakers.
· Presence of re-ground cuttings (LGS).
· Overpulls inside casing.
Freeing
See First Actions
Refer to Hole Cleaning section for more information.
- Tectonically Stressed Formations
DescriptionWell bore instability is caused when highly stressed formations are drilled and there exists a significant difference between the near well bore stress and the restraining pressure provided by the drilling fluid density. Tectonic stresses build up in areas where rock is being compressed or stretched due to movement of the earth’s crust. The rock in these areas is
being buckled by the pressure of moving tectonic plates. When a hole is drilled in an area of high tectonic stresses the rock around the well bore will collapse into the well bore and produce splintery cavings similar to those produced by over-pressured shale. In the tectonic stress case the hydrostatic pressure required to stabilise the well bore may be much higher
than the fracture pressure of the other exposed formations
This mechanism usually occurs:
· in or near mountainous regions.
Preventative action
Plan to case off these formations as quickly as possible. Maintain mud weight within planned mud weight window. Well bore instability shows itself as a hole cleaning problem. If possible drill these formations in smaller hole sizes. This will minimise the impact of a hole cleaning problem. Ensure that the circulation system is capable of handling the additional volume of cavings often associated with this mechanism. If hole problems do occur, Ref Hole Cleaning section. Use offset data to establish optimum inclination and azimuth as these are key factors in reducing the extent of the problem. Ref Wellbore Stability section in Rig site indicators
· Pack-offs and bridges may occur.
· Cavings at the shakers (splintery).
· Increase torque and drag.
· If stuck, circulation is likely to be impaired or non-existent.
· Increase in volume of returns at the shakers relative to the hole volume drilled.
Freeing
See First Actions